The invention relates to an apparatus for reproducing a binary digital signal from a track on a magnetic record carrier, comprising a read head for reading the digital signal from the track, which head has a signal output coupled to an input of a filter device, which device has an output coupled to an output terminal for supplying the digital signal, the overall response function of the read head and the filter device substantially corresponding to the response of a class-4 partial-response system, and the invention also relates to a read head intended for use in the apparatus.
An apparatus of the type defined in the opening paragraph is known from the publication "High data rate magnetic recording in a single channel" by C. H. Coleman et al, Journal of the IERE, Vol. 55, No. 55, No. 6, pp. 229-236, Jun. 1985.
Such a reproducing system, which is also referred to as a partial-response reproducing system, comprises an equalizer. This equalizer provides a correction for losses in the transmission channel. Transmission-channel losses are to be understood to mean write losses (losses occurring when the information is recorded in the magnetic record carrier) and read losses (the losses occurring when the information is read from the record carrier). The read losses may be classified as spacing losses (as a result of the spacing between the read head and the record carrier), tape-thickness losses, and gap length losses, for which reference is made to "The complete handbook of magnetic recording" by F. Jorgensen, p. 81. Since the frequency response characteristic for the gap length losses (which characteristic exhibits a sin x/x behavior) exhibits a spectral null at a wavelength equal to the gap length g, this means that the gap length of the read head in the known reproducing system is selected to be smaller than or equal to the bit length b of the bits in the digital signal recorded in the record carrier. This is because there are two bits per wavelength.
In the known reproducing system the filter device comprises two sections arranged in series. The first section is in fact that part of the equalizer which compensates for the gap losses of the read head. The second section is a Nyquist-2 filter or a class 4 partial-response filter. The response of the read head and the first section is equal to a constant (say 1). This means that the overall response of the read head and the filter device is in fact dictated by and is identical to the response of the second section.
The known reproducing system has the advantage that the level of the noise component in the digital signal being reproduced is low. A disadvantage is that intersymbol interference occurs during reading. This is only a minor disadvantage because it concerns controlled intersymbol interference for which a correction is possible. another reproducing system is known in which a Nyquist-1 filter is arranged after the read head, see for example the book "Digital and analog communication systems" by K. Sam Shanmugam, John Wiley 1979, in particular Chapter 5, pp. 189-203. In said system intersymbol interference does not pose a problem at the clock instants. However, as the bandwidth of a Nyquist-1 filter is larger than that of a partial-response system the amount of noise in the signal being read is larger than in the first prior-art reproducing system described herein.
An important parameter in recording and reproducing systems for digital signals is the information density on the record carrier. The information density on the record carrier should preferably be as high as possible, which means that the bit length of the signal recorded on the record carrier is sought to be made as small as possible. For example, it is envisaged that in the future bit lengths smaller than 0.25 .mu.m will have to be realized.
Write and read heads having a gap length (i.e. the length of the gap viewed in the longitudinal direction of the track) of 0.25 .mu.m or less are difficult to manufacture. Moreover, recording digital information in the record carrier by means of a write head having such a small gap length is very difficult as a result of head saturation. In addition, the efficiency losses during reading increase as the gap length decreases, for this refer to the afore-mentioned handbook by Jorgensen, pp. 86-94.